Wireless systems present certain difficulties in that they are often expected to operate in adverse environments that might contain interfering signals, reflections, indoor obstructions and the like. Problems in indoor environments include the “multipath” effect and the “fading” effect. The multipath effect is where an RF (radio frequency) signal emerging from a transmitter and arriving at a receiver propagates over two or more paths of sufficiently different lengths that different frequency components of the signal experience different attenuation and phase shift. The fading effect is where the contributions of the different paths to the RF signal at the receiver add together destructively for all frequency components of the signal.
A partial solution is the partitioning of available spectrum using orthogonal frequency division multiplexing (“OFDM”), which addresses some aspects of the multipath effect and fading by using substantially simultaneous transmission of a number of narrowband subcarriers that together occupy a substantially contiguous band of the radio frequency spectrum. In a particular OFDM approach, one that has been standardized under the IEEE 802.11a standard, 52 subcarriers are used and they are distributed symmetrically in the frequency domain about a common “carrier frequency”. Each subcarrier is modulated at a relatively low data rate and occupies a small bandwidth. The cumulative signal comprising all of the subcarriers is a relatively high data rate transmission whose total bandwidth is relatively large. In the case of 802.11a, it is approximately 16 MHz. The subcarriers can be modulated with binary phase shift keying (BPSK), quaternary phase shift keying (QPSK), quadrature amplitude modulation (QAM), or another keying.
In addition to begin organized into subcarriers in the frequency domain, signals in OFDM systems are distributed into symbols in the time domain. Each symbol undergoes a Fourier transform at the receiver to determine the amplitude of its subcarriers from which the encoded information is retrieved. Use of distinct symbols allows a more robust protocol. Taking advantage of the property that each subcarrier is periodic in time, with period equal to the duration of the symbol, each subcarrier can be extended by some additional fraction of its period and the overall symbol time extended so that any error at the receiver in determining the timing boundary between symbols does not result in overlap of adjacent symbols. Such a system is superior to simply leaving empty “guard intervals” between symbols, since the “cyclic extension” of the symbol subcarriers results in a simple phase shift in the frequency domain whose amplitude varies linearly with increasing subcarrier frequency.
In order to correctly recover encoded information from received 802.11 OFDM packets (e.g., 802.11a OFDM packets), the carrier frequency of each symbol should be determined. In addition, several other items should be determined, such as the linear phase shift imposed by imprecise symbol timing, the phase shift common to the subcarriers resulting from inconsistent receiver and transmitter oscillator frequencies (or imposed by bulk delay or multipath in channel propagation). These usually need to be determined with a relatively high accuracy in order to determine the baseband waveform with sufficient precision.
In order to facilitate this operation, the 802.11 OFDM protocol incorporates three techniques.
The first technique is a “short preamble”, which consists of a short, known waveform repeated 16 times, which is used to roughly estimate the carrier frequency of the incoming packet by computing the relative advance in phase from one repetition of the short waveform to the subsequent repetition.
The second technique is a “long preamble” that has greater duration and contains a longer waveform and which is repeated only twice. The carrier frequency estimate made with the short preamble is refined by determining the relative phase advance from the first repetition of this longer waveform, to the next.
The third technique is the use of pilot tones, wherein certain of the subcarriers are designated as pilot subcarriers. The phase of these subcarriers is set to known “pilot” values at the transmitter and does not bear payload information. Instead, known phase in the pilot subcarriers is compared with the phase of the pilot subcarriers measured at the receiver in order to determine the phase shift common to all the subcarriers of the symbol (all relative phase shift of the entire symbol) and can be used to track any advance in phase of the received packet after the timing determined from the long preamble is determined.
Because the frequency offset determined from the short and long preambles will have some small error, and the phase is derived from the frequency offset times time, then a phase error will steadily accumulate from one symbol to the next. If the entire sequence of symbols, comprising one packet, is longer than a certain duration this accumulated phase will prevent successful demodulation of the information carried in the symbols unless it can be accurately measured and compensated for.
Under certain propagation conditions, or when the receiver radio frequency is unstable (i.e., the frequency has not settled to its final value after turn-on, or the radio is subject to large phase deviations due to low-power operation, etc.), the phase that accrues as the packet is received may drift randomly, rather than advance or move in a predictable manner as it would if it were due entirely to a small error the estimate of the packet frequency offset.
In addition, the timing of the symbols comprising an OFDM packet is determined by the frequency reference employed by the transmitter and the receiver. When these frequency references differ, the timing of subsequent symbols at the receiver will appear to drift. Since timing drift manifests itself as a phase ramp in the frequency domain, timing drift appears as a linear ramp across the subcarriers comprising a given symbol.
The use of training symbols and pilot subcarriers to track phase changes at a receiver over time is known. See, for example, U.S. Pat. No. 7,184,495 to Thomson, et al. This can be a simple operation where the receiver knows in advance what training symbols were transmitted and what was transmitted on the pilot subcarriers. In instances where the signal is accompanied in the receiver by noise, the phase determined from the amplitudes of the pilot subcarriers may be averaged together to improve the accuracy of the phase measurement. The relative increase in phase from one subcarrier to the next may similarly be estimated by fitting a single-slope line to the estimated phase of the subcarriers in sequence. These estimates improve when more pilot subcarriers are available and used.
U.S. Pat. No. 7,039,004 describes channel tracking that uses data tones, but that requires knowing the symbols and dividing the symbols out from the signal before improving the channel estimation. U.S. Pat. No. 7,039,131 describes phase offset estimation that might use data-carrying subcarrier tones.
Some techniques for timing recovery are known. See, for example, M. Oerder et al., “Digital Filter and Square Timing Recovery”, IEEE Transactions on Communications, Vol. 36, No. 5 (May 1988), K. H. Mueller et al., “Timing Recovery in Digital Synchronous Data Receivers”, IEEE Transactions on Communications, Vol. COM-14, pp. 516-530 (May 1976) and F. M. Gardner, “A BPSK/QPSK Timing-Error Detector for Sampled Receivers”, IEEE Transactions on Communications, Vol. COM-34, pp. 423-429 (May 1986).
The number of pilot carriers allocated to the task of tracking the phase of the received packet is usually sufficient to track phase advance for signals that are received at a level high above the ambient noise level in the receiver, but for low signal levels (for instance, signals received from remote transmitters, or signals that have undergone destructive interference due to multipath propagation), the number of pilot carriers may be too small to track the changing phase and supplemental or alternative techniques might be needed.